Answer:
(During write-off) March 11
Dr    Bad debt expense                     $9,100
Cr    Accounts receivable                  $9,100    
(at the time of collection) March 29
  Dr Accounts receivable                      $9,100     
  Cr Bad debts expense                        $9,100
   
   Dr Cash                                              $9,100
   Cr  Accounts receivable                   $9,100 
Explanation:
On March 11, Dexter made an entry to write-off bad debts in the amount of $9,100. Dexter Co., charged it directly to Accounts receivable because the company uses direct write-off method. During the collection we have 2 steps to consider; First, On March 29 during the unexpected collection, Dexter shoud set up the reversal of the write-off entry which they had made last March 11. So we debit Accounts receivable and credit bad debts in the amount of $9,100. Second, is to record the collection, debit cash and credit Accounts receivable in the amount of $9,100.